Comment 5 for bug 1530383

Also, for the same reason, this piece of Artwork (ubiquity slideshows) isn't updated as also the wallpaper and other finishes aren't.

Look at the wallpaper piece: https://wiki.ubuntu.com/Lubuntu/Artwork/Incoming/Xenial/Box

You haven't this wallpaper on alphas because it's not uploaded yet. I guarantee, the usual procedure will "fix" the artwork elements as I did in previous releases.